The present invention relates to an image display apparatus capable of scrolling an image displayed on a screen by an interlacing method in a vertical direction and a display control method for scrolling the image.
Among display methods of a CRT (Cathode Ray Tube), a typical monitor apparatus for display of a television image, a computer image or the like on a screen, there is an interlacing (interlaced scanning) method which first scans an electron gun over only odd-numbered scanning lines from the top to the bottom of an image receiving screen and then scans over even-numbered lines. The interlacing method is known as a method that requires only half the number of lines scanned at a time by a non-interlacing (sequential scanning) method and also is capable of reducing flicker on the screen. The interlacing method is also used as a current television broadcast standard in Japan. Recently, functions of a television receiver have been diversified, and therefore a conventional method, for example use of buttons and other mechanical switches provided on the television receiver proper and a remote controller does not suffice to set the functions. Thus, the functions are often set by menu operation, which includes selecting a setting item from a menu frame displayed on the monitor screen and inputting into a displayed setting frame.
When an OSD (On Screen Display) image such as a menu image as mentioned above is displayed on a monitor apparatus using a CRT, the OSD image to be displayed is stored in a ROM (Read Only Memory) as digital image data, which is a set of pixels divided in a vertical direction and a horizontal direction, and then the OSD image is displayed by sequentially reading the digital image data in synchronism with scanning on the image receiving screen and supplying the digital image data to a video circuit under control of a processor. When a cursor, for example, as the OSD image is scrolled in a vertical direction on the screen, an address for reading image data from the ROM is changed by an amount corresponding to one scanning line.
When the image is scrolled in the vertical direction on the monitor screen employing the interlacing method, however, the image being scrolled may not appear to be moving smoothly. In the following, taking as an example an OSD character shown as a horizontal line image such as a cursor or a hyphen displayed over two scanning lines on the monitor screen employing the interlacing method, description will be made of movement of the image being scrolled. FIG. 9 shows change in the position of the OSD character being scrolled in a first conventional example.
In FIG. 9, the axis of abscissas designates passage of time t, whereas a solid line in a horizontal direction denotes a scanning line f1 in an odd-numbered field and a broken line denotes a scanning line f2 in an even-numbered field. Pixels forming the OSD character remain stationary on the screen in a time region indicated by T91, are being scrolled in an upward direction (scrolled up) in T92, and remain stationary in T93. Specifically, in timing G, the reading address of data representing the pixels of the OSD character is offset by an amount corresponding to two fields, that is, an odd-numbered field and an even-numbered field. Thus, the pixels over scanning lines f1 and f2 are both moved upward by two lines after the timing G. In timing I, the reading address is offset in the same manner, whereby the pixels are moved upward by two lines. Thus, in the timing G of offsetting the reading address in the time region T92 for scrolling up, there is a movement by three lines between the pixels in the even-numbered field f2 and the pixels in the odd-numbered field f1. In timing H, the pixels are moved by one line, whereas in the timing I, in which the reading address is offset again, the pixels are moved by three lines.
FIGS. 10A, 10B, and 10C show a displayed state of the OSD character in each of the timings in FIG. 9. FIG. 10A shows the case of the timing G; FIG. 10B shows the case of the timing H; and FIG. 10C shows the case of the timing I.
FIGS. 10A, 10B, and 10C show pixels 101, 102, and 103 being scanned in the current field in their respective timings within solidly shaded areas, and afterimages of pixels 104, 105, and 106 scanned in the previous field within diagonally shaded areas. As shown in FIGS. 10A to 10C, in conventional scrolling-up operation, when starting scrolling up the pixels during scanning of an odd-numbered field fl, or in the timing H, in which scanning is switched from the odd-numbered field f1 to an even-numbered field f2, the pixels 105 are moved to an adjacent scanning line of the pixels 102, and thus the pixels are scrolled smoothly. On the other hand, in the timing G and the timing I, in which scanning is switched from an even-numbered field f2 to an odd-numbered field f1, the pixels are moved to a position three lines away, that is, from the pixels 104 to the pixels 101 and from the pixels 106 to the pixels 103, respectively. This causes the scrolling of the display image to appear unnatural, and thus prevents smooth scrolling.
Such a problem can occur when the pixels are scrolled in a downward direction (scrolled down). This problem can occur when the pixels are scrolled down in timing in which scanning is switched from an odd-numbered field f1 to an even-numbered field f2, because in this timing, the pixels are moved to a scanning line three lines away, as described above. Therefore, the pixels are not scrolled smoothly.
The present invention has been made in view of the above problems, and it is accordingly an object of the present invention to provide an image display apparatus employing the interlacing method capable of smooth scrolling in a vertical direction.
It is another object of the present invention to provide a display control method employing the interlacing method capable of smooth scrolling in a vertical direction.
In order to solve the above problems, according to an aspect of the present invention, there is provided an image display apparatus capable of scrolling an image displayed on a screen by an interlacing method in a vertical direction, comprising: field determining means for determining whether a current field is odd-numbered or even-numbered; address calculating means for decrementing a reading address for reading image data in a vertical direction by a given number of lines at the start of scanning of an even-numbered field on the basis of a result of determining the field when scrolling the image in an upward direction and incrementing the reading address by a given number of lines at the start of scanning of an odd-numbered field when scrolling the image in a downward direction; and image data storage means for storing the image data and sequentially reading the image data according to the calculated reading address for output.
Such an image display apparatus determines whether the field currently being scanned is odd-numbered or even-numbered by the field determining means when scrolling in a vertical direction. When scrolling in an upward direction, the address calculating means decrements the address for reading image data from the image data storage means in a vertical direction by a given number of lines at the start of scanning of an even-numbered field, and then scrolling operation is performed, whereby the display pixels are moved upward by a given number of lines at the time of the scanning field switching. Similarly, when scrolling in a downward direction, the address calculating means increments the address for reading image data from the image data storage means in a vertical direction by a given number of lines at the start of scanning of an odd-numbered field, and then scrolling operation is performed, whereby the display pixels are moved downward by a given number of lines at the time of the scanning field switching.
In addition, according to another aspect of the present invention, there is provided a display control method for scrolling an image displayed on a screen by an interlacing method in a vertical direction, comprising the steps of: determining whether a current field is odd-numbered or even-numbered; decrementing a reading address for reading stored image data in a vertical direction by a given number of lines at the start of scanning of an even-numbered field on the basis of a result of determining the field when scrolling the image in an upward direction and incrementing the reading address by a given number of lines at the start of scanning of an odd-numbered field when scrolling the image in a downward direction; and sequentially reading the image data according to the calculated reading address for output.
In such a display control method, whether the field currently being scanned is odd-numbered or even-numbered is determined when scrolling in a vertical direction. When scrolling in an upward direction, the address for reading stored image data is decremented in a vertical direction by a given number of lines at the start of scanning of an even-numbered field, and then scrolling operation is performed, whereby the display pixels are moved upward by a given number of lines at the time of the scanning field switching. Similarly, when scrolling in a downward direction, the address for reading stored image data is incremented in a vertical direction by a given number of lines at the start of scanning of an odd-numbered field, and then scrolling operation is performed, whereby the display pixels are moved downward by a given number of lines at the time of the scanning field switching.